As disclosed in pending application Ser. No. 7/788,151, filed Nov. 5, 1991 now abandoned there are circumstances in which it is desirable to produce two different cooking liquors having different sulfidity in the production of cellulosic pulp, particularly in the production of kraft or sulfite paper pulp. The invention relates to a particular manner of effecting production of the two different cooking liquors, by forming two different melts in a soda recovery boiler. Utilizing this basic concept, it has been found that a wide diversity of pulp treatment procedures may then be employed, utilizing a wide variety of different types of cooking liquors. For example, it has been found that when it is practical to make different cooking liquors from different melts, under some circumstances it is not necessary to causticize the melts after they are dissolved, but rather they have a high enough sulfidity to use the liquor produced (e.g. green liquor) directly in the early stages of pulp cooking. Also, it has been found according to the invention that by oxidizing various pulp treating liquids, the alkali content thereof can be increased, which is useful both at the end of the digestion process, and in some subsequent treatment stages such as oxygen bleaching and alkali extraction. Alternatively, or in addition, it has been found that using a high sulfidity liquor, acid can be manufactured which is used in subsequent acidic treatments of the pulp, such as ozone bleaching.
The methods according to the invention are facilitated by utilizing a pulp mill having a recovery boiler with two different melt producing volumes. The conditions within the volumes may be kept separate; for example, one volume may have oxidizing conditions and the other reducing conditions. Also, the amount of sulfur in the waste gases that are discharged can be reduced by providing the low sulfidity melt producing volume above the high sulfidity melt producing volume so that sodium containing off gases from the low sulfidity melt producing volume will react with sulfur and the off gases from the high sulfidity melt producing volume to produce compounds that are more easily removed from the waste gas stream.
According to one aspect of the present invention, a method of recovering chemicals during the production of cellulose pulp using sulfur-containing chemicals is provided. The method comprises the following steps: (a) Providing a first fluid waste stream of sulfur-containing chemicals from the production of cellulose pulp, having a first sulfur content. (b) Providing a second fluid waste stream of sulfur-containing chemicals from the production of cellulose pulp having a second sulfur content, greater than the first sulfur content. (c) Combusting the first and second fluid waste streams separately to produce first and second melts, the second melt having the second sulfur content, and the first melt the first sulfur content. And (d) dissolving the first and second melts to produce first and second liquors, the first liquor having the first sulfur content, and the second liquor having the second sulfur content. The first and second fluid waste streams are preferably formed by heating a waste liquor (e.g. black liquor) to drive off sulfur-containing gases (such as DMS), recovering the sulfur from the sulfur-containing gases, splitting the heated waste liquor into the first and second different waste gas streams prior to step (c), and prior to or coincident with step (c) adding the recovered sulfur to only the second of the split streams.
Steps (a)-(d) are preferably practiced to produce a first liquor having a sulfidity of about 30% or less, and a second liquor having a sulfidity of about 40% or more, typically about 60-90%, and--where the second liquor is to be used uncausticized in early stages of kraft cooking--a sulfidty of about 70-90%. There is also preferably the further step (e) of adding other sulfur-containing constituents from the production of cellulose pulp to the second split stream prior to or coincident with the practice of step (c) for that stream. The constituents added to the second split stream may contain one or more of the following: flyash from the soda recovery boiler, waste acid from a tall-oil plant, waste acid from a chlorine dioxide plant, and sulfurous discharge gas from a pulp digester.
The first liquor may be treated with oxygen to increase the hydroxide ion concentration thereof, and then practicing a further step of, without intervening causticization treating the cellulose pulp in later stages of cook with the hydroxide-ion concentration enhanced first liquid. The first and second liquids may or may not be causticized depending upon the particular treatment sequences and the uses to which they will be put.
The invention also comprises a method of continuously producing cellulose pulp utilizing a continuous digester and first and second sulfur-containing cooking liquors, comprising the following steps: (a) Feeding comminuted cellulose material slurry entrained in the second sulfur-containing cooking liquor into the top of the continuous digester, the slurry moving downwardly in the digester during treatment. (b) Withdrawing black liquor from a first portion of the digester. (c) Below the first portion of the digester, withdrawing treatment liquor and recirculating it, and adding the first sulfur-containing cooking liquor to the withdrawn treatment liquor prior to recirculation. And (d) adding oxygen to the recirculated liquor. Step (d) is practiced to significantly raise the hydroxide ion concentration and thus the alkalinity of the slurry, and to assist in delignification. Typically the first cooking liquor has a lower sulfur content than the second cooking liquor (e.g. about 30% or below compared to about 50% or above). The black liquor withdrawn in step (b) is acted upon as by heating pursuant to the procedure disclosed in U.S Pat. No. 4,929,307 (the disclosure of which is hereby incorporated by reference herein) to produce two different fluid waste streams, which are used to produce two different melts as described above.
The invention also comprises a method of producing cellulose pulp from a slurry of comminuted cellulose material by the following steps: (a) Digesting the slurry by subjecting it to cooking with sulfur-containing cooking liquor at digesting conditions, black liquor being produced in the process. (b) Withdrawing black liquor produced during step (a). (c) Treating the pulp after step (a) in several alkali or acid treatment stages. (d) Producing cooking liquor from the black liquor withdrawn in step (b). (e) Oxidizing some of the cooking liquor from step (d) to increase the hydroxide ion concentration thereof. And (f) using the oxidized cooking liquor from step (e) to increase the alkali content during the practice of at least one of the alkali treatment stages of step (c). Step (c) includes oxygen bleaching and alkali extraction stages, and step (f) is practiced to add oxidized waste liquor to the oxygen bleaching and alkali extraction stages. At least a portion of the oxidized white liquor is recovered from step (f) utilizing countercurrent washing, and it is utilized in step (a). There may also be the step of utilizing a portion of the second melt or the second cooking liquor to produce acid, and then utilizing the acid in at least one acid treatment stage (e.g an ozone bleaching stage) during the practice of step (c). However, the acid can also be produced from sulfurous gases generated in the heat treatment of the black liquor.
According to still another aspect of the present invention, a method of kraft digesting comminuted cellulosic fibrous material is provided. The method comprises the steps of: (a) First treating the cellulosic material, in slurry form, with uncausticized green liquor having a sulfidity of about 70-90% at kraft digestion conditions, and then (b) treating the slurry with a second sulfur-containing liquor having a sulfidity of about 30% or less. Step (b) may be practiced with a causticized, oxidized, white liquid.
The invention also relates to a pulp mill and a soda recovery boiler. The pulp mill comprises: (a) Means for providing a first fluid waste stream of sulfur-containing chemicals from the production of cellulose pulp, having a first sulfur content. (b) Means for providing a second fluid waste stream of sulfur-containing chemicals from the production of cellulose pulp having a second sulfur content, greater than the first sulfur content. (c) Means for combusting the first and second fluid waste streams separately to produce first and second melts, the second melt having the second sulfur content, and the first melt the first sulfur content. And (d) means for dissolving the first and second melts to produce first and second liquors, the first liquor having the first sulfur content, and the second liquor having the second sulfur content. The means (c) preferably comprises a soda recovery boiler having at least two different melt sections and a common waste gas discharge, the first melt being produced in a first melt section, and the second melt in a second melt section. The first melt section may be vertically above and spaced from the second melt section, the first section discharging gases into the common waste gas discharge substantially above the discharge of gases from the second melt section into the common waste gas discharge. The first and second liquors may be separately clarified and causticized to produce a common supply of lime mud, and the lime mud may be fed to a lime reburning kiln to be calcinated.
The soda recovery boiler according to the invention comprises: A combustion chamber. Partition means for dividing the combustion chamber into first and second melt producing volumes. Means for adding a first fluid stream to only the first melt producing volume. Means for adding a second fluid stream to only the second melt producing volume. Means for withdrawing a first melt from only the first melt producing volume. Means for withdrawing a second melt from only the second melt producing volume. And a common waste gas discharge from the combustion chamber. The boiler according to the invention can have the modifications described above depending on the type of the pulp mill.
According to the present invention it is possible to produce pulp having a low kappa number, and good strength, with a high yield of pulp, and/or to minimize sulfur emissions from the mill. In view of the enhanced qualities of pulp produced according to the invention it is possible to eliminate chlorine and chlorine dioxide as bleaching chemicals, thus further minimizing any adverse environmental impact from the pulp mill.
The invention is particularly suited for combination with extended delignification kraft pulping techniques, such as the Kamyr, Inc. modified continuous cooking and extended modified continuous cooking techniques. In such techniques, about 60-80% of the total amount of white liquor is added at the beginning of the cook, while the rest is added at the end of the cook. By utilizing different sulfidity cooking liquors in the practice of these extended delignification kraft cooks, the results can be enhanced.
In sulphate cooking wood is treated with white liquor containing NaOH and Na.sub.2 S, so that lignin is dissolved and the cellulose fibers are released. The mixture of cellulose fibers (pulp) and cooking chemicals is treated with waster so that black liquor is generated. The black liquor is concentrated by evaporation. The concentrated black liquor is combusted in a soda recovery boiler and the chemical melt thus formed and mainly containing Na.sub.2 S and Na.sub.2 CO3 is dissolved into water, whereby green liquor is formed. The green liquor is then causticized with caustic lime (CaO) to white liquor containing NaOH. Another product of the causticization reaction is lime mud primarily being formed from CaCO.sub.3. The white liquor is supplied to a digester house and the lime mud is calcinated in a lime sludge reburning kiln to be reused as caustic lime during the caustization.
The pulp discharged from the digester may be, if desired, bleached. In order to decrease harmful environmental effects the cellulose pulping industry is striving to replace conventional chlorine bleaching with other alternatives, for example, by use of ozone. In ozone bleaching the pulp exiting the cooking apparatus is prebleached with oxygen under alkali conditions. Thereafter, the pulp is bleached with ozone, which is succeeded by alkali extraction. The alkali used in the oxygen bleaching and alkali extraction stage is oxidized white liquor. After each treatment the pulp is washed to remove the chemicals from the pulp, for example, by countercurrent washing. The washing agent from the last washing stage, to which the chemicals have been concentrated, is led to the recovery process of the chemicals.
The various detailed features of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.